Guide to the Meteograms

One very useful way to look at a time series of meteorological
data at a particular point is by ploting the data in a "meteogram." In a meteogram,
time progresses from left to right across the figure. One or more quantites
may be plotted, either as line graphs, bar graphs, symbols, numbers, shading,
etc. Using a meteogram, one can get a feel of how conditions change and evolve
at a stationary point (termed the "Eulerian" perspective). This is handy because
most people are rather stationary with respect to the atmosphere (unless you
do a lot of traveling), so a person's natural perspective is Eulerian. Weather
maps, or satellite "movies" are good for giving an overall perspective of the
"big picture", but it can be difficult to tell what will transpire in your particular
locality.

Keep in mind that like all the other model forecast information
provided by COLA, these data are "as is." They are straight from the computer
models at the National Centers for Environmental Research (NCEP) of the National
Weather Service. No interpretation, corrections, or other objective or subjective
changes have been made. These are not the "official" forecasts, though in most
cases they should resemble them rather closely since the official forecasts
for your region are based on these models to various extents.

Forecast meteograms are given for selected US cities.
In fact, the the forecasts are based on the grid point nearest the city in question.
The models cannot directly discern any features of the terrain or atmosphere smaller
than the grid resolution (although certain aspects such as the small-scale roughness
of the land surface, and the sub-grid scale distribution of thunderstorms are
represented indirectly by using parameterizations). Thus these models may not
do a good job of forecasting very localized weather such as might be associated
with things like mountainous terrain, or sea breezes.

Tropospheric Time-Height Cross Section

Conditions for the lower troposphere (up to 500 millibars)
are shown in profile. The model data we receive is interpolated down to 1000 millibars,
but over high terrain only data that is near or above ground level is displayed.
Data below ground level has no physical meaning and is omitted.

Winds

The barbs indcate the direction and speed of the wind.

The units are MPH for the US cities, and m/s for Canada.

A full barb = 10, and a short barb = 5. A pennant
= 50. The speed can be found by tallying the barbs.

Counter to intuition, the barbs are not like the tail
of a wind vane, but rather project into the wind. A stem pointing left
with one full barb and one half barb indicates a wind from the west at
15.

Temperature

The colored contours indicate the profile of temperature.

The units are °F for the US cities and °C
for Canada. The contour interval is 10°F or 5°C.

The freezing level (32°F or 0°C) is indicated
by the double black line labeled FR.

Relative Humidity

The graduated green shading indicates relative humidity.

The units are percent.

1000-500 mb Thickness

Thickness is the vertical distance between two pressure
levels. In general it is true that the distance will be a function of the density
of the air between the two pressure levels, which is itself directly related to
the temperature of the air. Thus, thickness is a good indicator of the mean temperature
in the layer of atmosphere between the two levels -- greater thickness = warmer
air.

The cyan line shows the thickness of the 1000-500 millibar
layer of the atmosphere. This is approximately the bottom half of the atmosphere
(except over high terrain).

The units are dekameters (10s of meters, 10 meters is
about 33 feet).

One rule of thumb is that if the thickness of this layer
is less than 540 dm, that any precipitation will be in the form of snow.

Stability Indices

The stability indices are measures of the potential for
strong or severe weather. The indices shown here are the Lifted Index (LI) and
the Convective Available Potential Energy (CAPE).

LI

The LI, indicated by the red line, is a measure of
the thunderstorm potential which accounts for low level moisture availability.

LI values greater than 0 mean thunderstorms are unlikely

LI values between 0 and -2 mean thunderstorms are
possible with good trigger

LI values between -3 and -5 mean thunderstorms are
probable

LI values less than -5 mean a strong potential for
severe thunderstorms

CAPE

The purple bars indicate the CAPE value at the surface in units of J/kg.

CAPE is a measure of the buoyancy of a layer. The larger the CAPE, the greater the potential for severe weather.

Any value greater than 0 J/kg indicates instability and the possibility of thunderstorms.

The base line for the bar graph of CAPE is 0.

Sea Level Pressure

Sea level pressure (SLP) is the surface pressure interpolated
down to sea level from the altitude of the grid box of the model. This corresponds
to the barometric pressure one hears reported on local radio or TV weather reports.

The blue line indicates SLP.

The units are millibars.

Valleys in SLP often indicate frontal passages, and will
often coincide with pronounced changes in wind direction, temperatures, and
humidity.

In subtropical regions (the deserts of the Southwest,
and much of the Sunbelt during summer) the SLP often oscillates daily, with
a peak in the early morning and a trough during mid-afternoon.

2-meter Temperatures

Air temperature and dew point temperature are given at
the 2-meter level (6½ feet above ground). The dew point temperature is the
temperature that a sample of air would have if it was cooled (at constant pressure)
until it reached saturation. The dew point temperature is an alternative way to
describe the amount of moisture or humidity in the air. If the dew-point temperature
is close to the air temperature, the relative humidity is high, and if the dew
point is well below the air temperature, the relative humidity is low. One technique
for forecasting overnight low temperature is to look at the daytime dew point:
if no fronts are expected to come through, tonight's low temperature will not
get much below today's dew point.

Air temperature is indicated by the red line (with color
shading below the line to aid interpretation).

Dew point temperature is indicated by the grey line.

The units are °F for the US cities and °C for
Canada.

The freezing level is drawn with a dot-dashed black line.

Temperatures at the specified times are given, so low
and high temperatures may fall between the hours specified, and thus exceed
the range shown.

2-Meter Reletive Humidity

The green line and graduated green shading indicates relative
humidity.

The units are percent.

Cloud Cover

This panel has a blue background to show the cloud-free areas. The panel is divided into three horizontal layers for the display of low, middle, and high cloud cover, which are drawn as white bars. If the white bar covers the full height of its layer, that is 100% cloudiness. The white bars have no gap between them to better simulate the appearance of cloudiness in the panel.

Precipitation

The bar graph at the bottom of the figure indicates predicted
precipitation types and amounts.

Precipitation Type

The color inidicates the type of precipitation: rain,
sleet, snow, or ice pellets.

Narrow red bars within the other bars indicates the
portion of precipitation likely to come from convection (showers and thunderstorms)

Precipitation Amount

Units in inches for US cities, centimeters for Canada.

Values are accumulated precipitation over the previous
forecast period, so no precip values are given at hour 00.

The amounts are liquid water equivalents. For
example, 1" of snow means one inch of water in the melted snow. It could
mean anywhere from 3-12" of actual snow, depending on how "wet" the snowfall
is. Depths of sleet and ice will more closely match their liquid water
equivalents.